I see many levels to this question. From raw chromosome evaluation, two. BUT... if you do a web search of this phrase: homosexual + brain scan - you would find UK research from the 1990s using positron emission tomography (P.E.T. scans that can map the living brain). They discovered that there is such a thing as a "male brain" or a "female brain" from a structural viewpoint. That is, physically detectable differences exist between the two configurations. I'm not an anatomist or neurosurgeon so don't ask me for the details.
The earliest research found the differences, but subsequent research noted that some subjects had male brains in female bodies and vice versa. Upon questioning, such people admitted that they were homosexual. In these cases, the wiring didn't match the plumbing, and the wiring won. So now you have four cases, the combination of either of two brains and either of two sets of plumbing. The researches also found some ambiguous cases where the brain was not clearly in either configuration.
This makes me believe that there must be a spectrum of brain configurations. Remembering that the brain is the most powerful of all of your sex organs (because it is the thing that responds to arousal stimuli), this would explain a lot of the variations that have come to light in the past years. If there is a spectrum of brain configurations, then one might posit a spectrum (or at least a range) of gender preferences and identities.
This also puts the typical religious response into question. Religious fundamentalists claim that homosexuality is a choice, but in New Orleans as a Bourbon Street musician, I met people of every variation. NONE repeat NONE of the gay people thought of their situation as a choice.
One of my best friends in college, a woman whom everyone thought I would marry, "came out" to me one day in tears because she was catching flak from her own sister, who didn't even know her own sister's issues because they had become estranged. All I asked her was "Can we still be friends?" and that led to more tears - following her answer of "Yes." We became really serious as a duplicate bridge partnership. We were decent on bidding but sharks on defense. When we talked about her gender issues, she told me that from about age 13, it stopped being a choice for her. Then the only issue between us was that we sometimes lusted after the same woman.
Then I got married. A few years later (maybe 5-8 years) I learned that my step-daughter P. was gay. When the USA Supreme Court dropped the gavel on the anti-gay laws regarding marriage, P. and her partner T. got married. When I talked with them, neither one thought of their preferences as a choice. They had previously tried to conform to that strict societal viewpoint about such issues but found that they could not. The unbearably rigid way in which some churches think of gays proved to be more than they could handle and they changed denominations to find an inclusive church.
The brain/genitalia combination ALSO explains the common statement made by some gays, particularly the transvestite ones, that they felt they had been born into the wrong body. If they had a mismatch between wiring and plumbing, they actually might have been.